CASSITERITE AFTER FELDSPAR

The following article is aimed at providing some general notes on cassiterite pseudomorphs after feldspar.

Mineral collectors would be aware of the famous pseudomorphs found at Wheal Coates (Huel Coates) in Cornwall, but this is not a unique locality. A reading of the literature indicates that while localities from which cassiterite pseudomorphs after feldspar have been found are relatively few there are at least a handful around the world.



Are these pseudomorphs in fact pseudomorphs?

It would appear that most of these pseudomorphs are of Orthoclase feldspar although I note that there is some doubt that they are all Orthoclase. JM Coon ^[1] refers to a specimen “probably orthoclase or microcline”. I am not aware of any definitive study of the feldspar.

It would also appear that these "pseudomorphs" may not be true pseudomorphs in that they are casts. JM Coon indicated that “there is no doubt that the pseudo’s are infillings of the matrices formerly filled with feldspar. Embry & Symes ^[2] also referred to these as being casts. This helps explain why the majority of these all have a fine sand grained appearance.

Joseph Carne ^[3] theorised on the formation of these pseudomorphs, commenting that “the tin has here taken the place of the felspar can scarcely be doubted, as the specimens .... exhibit the change in almost all its stages. It is probable, however, that previous to the replacement by the tin, the felspar had become decomposed, and by giving out its alkali had remained in the state of a sandstone. [Describing some particular specimens]... Here is first the crystal of felspar, or rather sandstone, partially decomposed; another crystal shows that the tin has just begun to invade it; in another, it has made greater progress; two others are nearly equally divided — one lengthwise, the other crosswise — in which one half is oxide of tin, and the other half felspar; in another, only a little felspar remains at one end of the crystal; and others exhibit the complete replacement, and even a new crystallization, of tin on the first crystal. In a broken crystal, the fracture, lustre, etc., are just as they appear in the common rich tinstone of our mines.”

Cassiterite pseudo Orthoclase (Carlsbad & Baveno twin xls) - Wheal Coates

Cassiterite pseudo Orthoclase (Carlsbad & Baveno twin xls) - Wheal Coates

Joseph Collins ^[4] commented that “large pseudomorphous crystals of Cassiterite after felspar occurred formerly in considerable abundance at Huel Coates, St. Agnes,... Collins commented further that “many of the larger crystals "presented the curious circumstance of shewing the original felspar replaced by oxide of tin only about half across longitudinally." It is clear from this reference that many of the pseudomorphs were only partially transformed. If these are not regarded as true pseudomorphs, but rather replacements, I find it unusual that only half a feldspar crystal is "dissolved" and the half-caste filled with cassiterite.

CORNWALL

Wheal Coates (Huel Coates)

This mine is located near St Agnes Beacon in Cornwall. The cassiterite pseudomorphs were stated as being found in the vicinity of two intersecting lodes called the “Sand lode” and “Great lode” about 330 to 450 feet below ground.

JM Coon ^[5] describes the pseudomorphs as being generally of “feldspar crystals, usually Carlsbad in type”. The pseudomorphs may be of single crystals, Carlsbad twins, Manebach twins and Baveno twins and as JM Coon describes “intricate macles”. This reference to macles is simply to the complex clusters of twinned pseudomorph crystals in general.

Cassiterite pseudo Carlsbad twinned Orthoclase cluster - Wheal Coates

The pseudomorphs range in size up to 7.5cms in length with the majority ranging in sizes up to 2.5cms.

Cassiterite pseudo Carlsbad twinned Orthoclase cluster - Wheal Coates

There are reports that they were found in their “millions’ but this may well have been an exaggeration. FW Rudlee ^[6] mentions “the lode which yielded these remarkable pseudomorphs, which are said to have "occurred in millions," was in a soft decomposed granite, or perhaps an elvan [a quartz-porphyry], and contained so much sandy matter that it has been called a "sand lode".

Apparently the actual tin content of these pseudomorphs varies anything up to 70%, so this would account for the majority of these pseudomrphs simply going to the crushers.

Cassiterite pseudo Orthoclase - Single xl and Carlsbad twinned xl - Wheal Coates

Other Cornwall localities

Collins ^[7] states that similar cassiterite pseudomorphs of feldspar also were found at:

• Carn Brecon (Cam Brea), St. Mewan, near St. Austell; and
• Balleswidden, St. Just (around 1861).

A further locality is noted by Hosking and Camm ^[8]

• Cameron (Beacon) Quarry, St Agnes.
This quarry is situated at the south-western end of the St Agnes/Cligga Head granite ridge (National Grid Reference: SW 704507).

Cornish Fakes

It is worth noting that despite the fact that alleged “millions” of these pseudomorphs were found, fakes are known. Why they were made is unclear, but perhaps the larger ones were relatively rare and sought after. These fakes were made using lead sheets with fine cassiterite sands embedded thereon. It would appear that these were made in the late 1800s. These may well have been some of the first mineral fakes ever made. A photograph of one of these appears in the text, Minerals of Cornwall and Devon.^[9]

AUSTRALIA

Little known by most collectors is the fact that specimens of cassiterite after feldspar have also been found in Australia, notably:

• Mount Bischoff, Tasmania
• Rex Hill Mine (Mount Rex Mine), Rossarden district, Tasmania
• Stanley River tinfield (Zeehan district), Tasmania

WP Pettard ^[10] commented on a find of partial pseudomorphs as a “remarkable occurrence of cassiterite has recently been discovered at Mount Bischoff by Mr. Bradford, in which cellular cavities, retaining in most instances the form of orthoclastic felspar, have been lined, and sometimes completely filled, with a mass of minute, well-developed, tetragonal tin crystals, combined with pycnite [Pyknite]. The cavities are small, but perfectly distinct. The original porphyry has been almost completely topazised, simply leaving granular interspersed quartz.

Yet another find of these pseudomorphs, only much more similar to those found in Cornwall, was at the Rex Hill Mine (Mount Rex Mine), Rossarden district, Tasmania. Pettard commented that the pseudomorphs from here represented those from “where pseudomorphism has gone to the fullest extent”.

Cassiterite pseudo Carlsbad twinned Orthoclase - Rex Hill Mine

At the Stanley River tinfield (Zeehan district), Tasmania, Pettard ^[11] describes a find of “remarkably fine pseudomorphs of compact black cassiterite after orthoclase”. He describes them as having a thin replacement that clearly showed the distinct crystal outline of the feldspar, which in turn was “imbedded in an almost milk-white porphyritic rock, mainly composed of quartz and felted masses and radiating bunches of short, and opaque crystals of tourmaline”.

BOLIVIA

Alfredo Petrov, via personal communication, has advised me that cassiterite pseudo orthoclase specimens have been found at Llallagua, Bolivia. I note that Alfredo and Jaroslav Hyrsl co-wrote an article on pseudomorphs from Bolivia in Rocks and Minerals in 1998. ^[12]

Photographs and specimens

All specimens in this article were acquired by me over many years. I find each of them absolutely fascinating.

Footnotes

1.	Pseudomorphs of Cassiterite, etc., after Feldspar at Wheal Coats, St. Agnes, Cornwall
2.	Embrey, PG & Symes RF, Minerals of Cornwall and Devon
3.	On the Pseudomorphous Minerals found in Cornwally illustrative of a replacement of one mineral substance by another
4.	A Handbook to Mineralogy of Cornwall and Devon with instructions for their discrimination and copious tables of localities, 1871 Part II p 25
5.	Pseudomorphs of Cassiterite, etc., after Feldspar at Wheal Coats, St. Agnes, Cornwall
6.	F W Rudlee, "A handbook to a collection of the minerals of the British Islands, mostly selected from the Ludlam collection, in the Museum of Practical Geology" 1904
7.	A Handbook to Mineralogy of Cornwall and Devon with instructions for their discrimination and copious tables of localities, 1871 Part II p 25
8.	Hosking, KF and Camm, GS 1985, The occurrence of cassiterite and other species of economic interest in the greisenized granite porphyry of Cameron Quarry, St Agnes, Cornwall
9.	Embrey, PG & Symes RF, Minerals of Cornwall and Devon
10.	Pettard, WP, Notes on Tasmanian Minerals, Royal Society Paper
11.	Pettard, WP, Notes on Tasmanian Minerals, Royal Society Paper
12.	Jaroslav Hyrsl, Alfredo Petrov Pseudomorphs from Bolivia - A Review, Rocks & Minerals Vol 73 November/December 1998, p410-414

References

Coon, JM: Pseudomorphs of Cassiterite, etc., after Feldspar at Wheal Coats, St. Agnes, Cornwall

Carne, Joseph: On the Pseudomorphous Minerals found in Cornwally illustrative of a replacement of one mineral substance by another, 1841, Paper read to the Royal Geological Society

Embrey, PG & Symes RF: Minerals of Cornwall and Devon, British Museuem (Natural History), London 1987 p.69

Collins, JH: A Handbook to Mineralogy of Cornwall and Devon with instructions for their discrimination and copious tables of localities, 1871 Part II p.25.

Pettard, WP: Notes on Tasmanian Minerals, Royal Society Paper. Secretary for Mines Report for half year ending 31st December 1903.

Rudlee, FW: "A handbook to a collection of the minerals of the British Islands, mostly selected from the Ludlam collection, in the Museum of Practical Geology" 1904

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